An abbreviation which stands for the trigonometric function, "cosine." In its native mathematical context, cosine is used in solving for side lengths or angle measurements in right-triangles, exclusively.
In regards to its place within the acronym: SOHCAHTOA, cosine occupies the "CAH," wherein the series forms: Cosine (equals) Adjacent (over) Hypotenuse.
When dealing with an angle measurement, x ("theta"), the side "touching" the angle is referred to as the Adjacent side; the side furthest away from the angle is referred to as the Opposite side; and, in a right triangle, the hypotenuse always remains and, simplistically, may be recognized as the diagonal side.

Note: UrbanDictionary entries do not support Entity, Hex or Decimal browser rendering. This definition of "cos" uses an "x" in place of the Greek small letter, "theta".
However, in reality, theta appears as an "o" or a "zero" with a line going horizontally through the center.